1. Field
The following description relates to organic light emitting display devices.
2. Discussion of Related Art
Recently, various flat panel display devices capable of reducing weight and volume, which are disadvantages of cathode ray tubes, have been developed. Among the flat panel display devices, there are liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices, etc.
Among the above discussed flat panel display devices, the organic light emitting display devices display images using organic light emitting diodes that generate light by the recombination of electrons and holes. Organic light emitting display devices are driven at low power consumption, with rapid response speed.
FIG. 1 is a schematic circuit diagram showing a conventional pixel of an organic light emitting display device. In FIG. 1, the transistors included in the pixel are NMOS transistors.
Referring to FIG. 1, the pixel 4 of the conventional organic light emitting display device includes a pixel circuit 2 that is coupled to an organic light emitting diode OLED, a data line Dm, and a scan line Sn to control the organic light emitting diode OLED.
The anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2, and the cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS. The organic light emitting diode OLED generates light having a brightness (e.g., a predetermined brightness) corresponding to the current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED according to the data signal supplied to the data line Dm and a scan signal supplied to the scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 (i.e., a driving transistor) coupled between a first power supply ELVDD and the organic light emitting diode OLED, a first transistor M1 coupled between the second transistor M2, the data line Dm, and the scan line Sn, and a storage capacitor Cst that is coupled between the gate electrode and a first electrode of the second transistor M2.
The gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode of the first transistor M1 is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Here, the first electrode of the first transistor M1 is either a source electrode or a drain electrode, and the second electrode of the first transistor M1 is an electrode other than the electrode of the first electrode. For example, if the first electrode is the source electrode, the second electrode is the drain electrode. When the scan signal is supplied to the scan line Sn, the first transistor M1 coupled between the scan line Sn and the data line Dm is turned on to supply the data signal supplied from the data line Dm to the storage capacitor Cst. Thus, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and the first electrode is coupled to the first power supply ELVDD. The second electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and is also coupled to the anode electrode of the organic light emitting diode OLED. The second transistor M2 controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS via the organic light emitting diode OLED in accordance with the voltage stored in the storage capacitor Cst.
One terminal of the storage capacitor Cst is coupled to the gate electrode of the second transistor M2, and the other terminal of the storage capacitor Cst is coupled to the anode electrode of the organic light emitting diode OLED. The storage capacitor Cst is charged with the voltage corresponding to the data signal.
A conventional pixel 4 as described above supplies a current corresponding to the voltage charged in the storage capacitor Cst to the organic light emitting diode OLED, thereby displaying an image having a brightness (e.g., a predetermined brightness). However, an issue with this conventional organic light emitting display device is that it cannot display an image having a uniform brightness due to the deviation of the threshold voltage of the second transistor M2.
Actually, when the threshold voltage of the second transistors M2 are different in the respective pixels 4, the respective pixels 4 generate light having different brightness corresponding to the same data signal, and the conventional organic light emitting display device cannot display an image having a uniform brightness.